An ignition device mounted in a spark ignition type internal combustion engine causes a spark discharge and ignition to occur between a central electrode and a ground electrode of an ignition plug by applying a high voltage generated in an ignition coil when an igniter is turned off to the central electrode of the ignition plug.
Recently, in order to attain a stable flame by ensuring ignition of an air fuel mixture present in a combustion chamber of a cylinder, there is an attempt to perform an “active ignition” method of emitting a high frequency wave outputted from a high frequency oscillator or a microwave outputted from an electric field generation circuit, i.e., a magnetron, into the combustion chamber (see, for example, Japanese Unexamined Patent Application, Publication No. 2011-159477 and Japanese Unexamined Patent Application, Publication No. 2011-064162). According to the active ignition method, it is possible to create an electric field of a high frequency wave or a microwave in a space between a central electrode and a ground electrode, and enlarge plasma generated in the electric field to a large flame kernel followed by flame propagation combustion.
Meanwhile, in a case in which the air fuel mixture is insufficiently combusted resulting from the fact that the flame is weakened or the like in the process of combustion, a gas containing an unburned fuel component is exhausted from the cylinder to an exhaust passage, and reaches a three way catalyst for cleaning the exhaust gas. Consequently, there is a concern that a spontaneous ignition (after-fire) of the fuel component may occur in a high temperature part of the exhaust passage, or oxidization of the fuel component may occur in the catalyst, thereby excessively increasing the temperature of the catalyst and causing the catalyst to be melted.